It is known to transmit power over differential data lines to power remote equipment. Power Over Ethernet (PoE) is an example of one such system. In PoE, limited power is transmitted to Ethernet-connected equipment (e.g., VoIP telephones, security cameras, etc.) from an Ethernet switch. The Ethernet switch is the Power Sourcing Equipment (PSE). DC power from the switch is transmitted over two sets of twisted wire pairs in the standard CAT-5 cabling. One wire pair conducts the positive voltage while the other wire pair conducts the reference voltage (e.g., system ground). One or both of the wire pairs may also transmit and receive differential data signals, and the DC voltage is applied as common mode. In this way, the need for providing any external power source for the Powered Devices (PDs) can be eliminated. The standards for PoE are set out in IEEE 802.3, incorporated herein by reference. The CAT-5 cable has four twisted wire pairs, and two of the wire pairs are typically not used.
In such prior art systems, the power supply has one output (e.g., the positive voltage terminal) connected to only one node of a first DC coupling circuit that provides the positive voltage to a first wire pair, and has its other output (e.g., ground) connected to only one node of a second DC coupling circuit that provides the reference voltage on a second wire pair. One or both of the wire pairs also conducts differential data.
Since the power is coupled to only one node for each wire pair, all current flows through that one node. Therefore, all components connected in series between that node and the wire pair must carry the full PD current, so those components must be designed to safely conduct the maximum PD current.
In some cases, the power is transmitted through a magnetic component such as a center-tapped isolation transformer, an autotransformer, inductors, and/or a common mode choke (CMC). The current-carrying capacity of such a PoE system is often limited by the current-carrying capacity of the magnetic components. Due to constraints in magnetic design, it may not be feasible to increase the current-carrying capacity of these devices beyond a certain limit. Below are some of the design constraints on the magnetic components.
A center tapped transformer presents a load to the differential data. This loading effect is typically characterized as the transmitter droop specification or the return loss and needs to be better than the worst-case limit specified in the corresponding standards for reliable communication. This dictates some design constraints on the center tap transformer which include a minimum Open Circuit Inductance (OCL) and Self Resonant Frequency (SRF). Also, since the transformer needs to magnetically couple the differential data signal from its primary winding to its secondary winding, it needs to have low leakage inductance which can otherwise adversely affect the return loss. The transformer also provides galvanic isolation and needs to be able to survive a high voltage across the isolation barrier. Considering these requirements, the transformer needs to have a minimum number of winding turns, a low enough parasitic capacitance, a tight enough magnetic coupling, and a high breakdown potential for the winding insulation. With these and other constraints (e.g., on the core material), increasing the current capacity of the transformer may not be technically and economically feasible beyond a certain value. Similar design constraints are applicable to the other magnetic components as well.
A CMC ideally presents no load to the differential data. This requires that it is a tightly coupled (high magnetic-coupling coefficient) device with low leakage and a low inter-winding capacitance. The CMC, however, needs to present a high impedance to the common mode (CM) signals. This dictates a minimum Open Circuit Inductance (OCL) and a minimum Self Resonant Frequency. It is noteworthy that different parasitic capacitances get excited (and become relevant) when measuring the Differential Mode Return Loss or the Common Mode Insertion Loss. These restrictions on the CMC design also mean that increasing the current capacity of the CMC may not be technically and economically feasible beyond a certain value.
What is needed is a new PoE technique that can supply more power to the powered device without requiring the magnetic components to be designed with an increased current capacity.